Control of cardiac output Flashcards
what is the definition of cardiac output?
→amount of blood ejected from the heart per minute
what is the equation for cardiac output?
→CO = HR x SV
what is the equation for blood pressure?
→BP = CO x TPR (total peripheral resistance)
what does cardiac output determine?
→blood pressure
→blood flow
what is cardiac output proportional to?
→ how often the heart beats PM
→ how much blood is ejected per beat
what controls stroke volume? (list)
→ preload
→ heart rate
→ contractility
→ afterload
what is the equation for blood flow (CO)?
CO = BP/TPR
what is preload?
→ The stretching of the heart at rest
→ Increases stroke volume due to Starling’s Law
what is afterload?
→ Opposes ejection
→Reduces stroke volume due to Laplace’s law
what controls heart rate?
→ sympathetic and parasympathetic nerves control heart rate
What is energy of contraction and what does it depend on?
→ amount of work done required to generate stroke volume
→ depends on Starling’s Law and contractility
What two functions does stroke work carry out?
→ Increases chamber pressure to make it greater than aortic pressure (isovolumetric contraction)
→ Ejection from the ventricle
what do afterload and preload do to the stroke volume?
→ Preload increases the stroke volume and afterload opposes the stroke volume
State Starling’s law
→ Energy of contraction in cardiac muscle is relative to the muscle fibre length at rest
→the greater the stretch of the ventricle in diastole (blood entering)
→the greater the energy of contraction
→ a greater stroke volume is achieved in systole
what is the equation for stroke volume?
→ SV = end diastolic volume - end systolic volume
Describe an unstretched fibre
→ overlapping actin/myosin
→ mechanical interference
→ Less cross-bridge formation available for contraction
Describe a stretched fibre
→ Less overlapping actin/myosin
→ Less mechanical interference
→ potential for more cross-bridge formation
→ Increases sensitivity to Ca2+ ions
What are the roles of Starling’s law during preload?
→ Balances the output of the right and left ventricle which is very important
→ responsible for the fall in cardiac output during a drop in blood volume
→ Restores cardiac output in response to IV fluid transfusions
→ Responsible for fall in cardiac output during orthostasis leading to postural hypotension + dizziness
→ contributes to increased stroke volume and cardiac output during upright exercise
What is afterload determined by?
→ it is determined by wall stress directed through the heart wall
→stress through the wall of the heart prevents muscle contraction
What is the equation for Laplace’s law?
→ P = 2t/r
T = wall tension
P = pressure
R = radius
what is the equation for wall tension?
→ T = S (wall stress) x ( wall thickness) W
How is afterload increased and how it it reduced ?
→ by increasing pressure + radius and reduced by increasing wall thickness
what happens if there is a small radius (in terms of afterload)?
→ greater wall curvature
→ more wall stress directed towards the center of the chamber
→ less afterload
→ better ejection
what happens if there is a big radius (in terms of afterload)?
→ less wall curvature
→ more wall stress directed through heart wall
→more afterload
→ less ejection